What I've seen in the past is that the rogue party can register old people or those groups known to avoid voting in the past and therefore acquire valid private keys. The few that want to vote would ask for the private key and a new one would be provided to them. The other topic is private key interception, copying them in transit to the recipient. You are checking through the pub keys who has voted and then injecting the votes on election night when it becomes obvious who isn't going to vote on that event. Voting in presence with a picture taken would solve these identity issues. There would always exist some fraud, just difficult to scale into larger numbers.